こんにちは TAG-さん!

I'm requesting a TAG review of CSS Values and Units: sibling-count() and sibling-index().

The CSS Values and Units Module Level 5 introduces tree counting functions, specifically sibling-index() and sibling-count(). These functions can be used as <integer> values directly or combined with other values in calc() expressions. The sibling-index() and sibling-count() functions evaluate to integers at computed value time based on the flat tree position of the element.

• Explainer¹: https://github.com/w3c/csswg-drafts/blob/main/css-values-5/tree-counting-explainer.md
• Specification: https://drafts.csswg.org/css-values-5/#tree-counting
• WPT Tests: https://wpt.fyi/results/css/css-values/tree-counting
• User research:
• Security and Privacy self-review²:
• GitHub repo: https://github.com/w3c/csswg-drafts
• Primary contacts:
  • @lilles, Google, Implementer
  • @tabatkins, Google, Editor
• Organization/project driving the specification: CSSWG
• Multi-stakeholder support³:
  • Chromium comments: Implementation in progress behind a flag
  • Mozilla comments: https://github.com/mozilla/standards-positions/issues/1194
  • WebKit comments: https://github.com/WebKit/standards-positions/issues/471
  • Links to articles in the explainer
  • Some developer reactions: bsky
• Status/issue trackers for implementations⁴:
  • https://chromestatus.com/feature/6225478530367488
  • https://crbug.com/40282719
  • https://bugzilla.mozilla.org/show_bug.cgi?id=1953973

Further details:

• I have reviewed the TAG's Web Platform Design Principles
• The group where the work on this specification is currently being done: CSSWG

You should also know that there are open issues to extend tree-counting functions:

• Additional support for light tree counting
• Add selector argument
• Add children-count() function
• Add descendant-count() function